Technical Field
The present invention relates to a spark ignition engine, and in particular to a spark ignition engine in which a thermal insulation layer is formed on a wall surface, facing an inside of a combustion chamber, of at least a part of base materials forming the combustion chamber of the internal combustion engine.
Related Art
In order to improve thermal efficiency of an internal combustion engine, a technique has been proposed in which a thermal insulation layer is formed on a wall surface, facing an inside of a combustion chamber, of at least a part of base materials forming the combustion chamber of the internal combustion engine (for example, refer to JP 1-142246 A). In JP 1-142246 A, for example, a thermal insulation layer made of ceramic with a base material of, for example, zirconia is formed on a surface of components of the combustion chamber such as a valve face, a cylinder head surface, a cylinder wall, and a piston top surface, to a thickness in a range of 50 μm-220 μm. With this configuration, an amount of transfer of heat from the gas in the combustion chamber to the components of the combustion chamber is reduced, and the thermal efficiency is improved.
A heat loss Qloss [W] in the cylinder of the internal combustion engine can be represented by the following equation (1), with a heat transfer coefficient h [W/(m2·K)] depending on a pressure in the cylinder and the gas flow, a surface area A [m2] in the cylinder, a gas temperature Tg [K] in the cylinder, and the temperature of the wall Twall [K] facing the inside of the cylinder (that is, contacting the gas in the cylinder):Qloss=A×h×(Tg−Twall)  (1)
During the cycles of the internal combustion engine, the gas temperature Tg in the cylinder changes with time, but by changing the wall temperature Twall with time so as to follow the gas temperature Tg in the cylinder, the value of (Tg−Twall) in equation (1) can be set small, and the heat loss Qloss can be reduced.
When the thermal insulation layer is formed on a wall surface facing the inside of the combustion chamber of the internal combustion engine, if the thickness of the thermal insulation layer is increased, the amount of transfer of heat from the gas in the combustion chamber to the components of the combustion chamber can be reduced, and consequently, the thermal efficiency of the internal combustion engine can be improved. However, when the thickness of the thermal insulation layer is increased too much, the temperature of the wall of the combustion chamber which has increased during the combustion stroke is not sufficiently reduced during the expansion and exhaust strokes, and would be at a higher temperature at the start of the intake stroke in the next cycle compared to the configuration without the thermal insulation layer, resulting in an increased amount of heat being transferred from the wall of the combustion chamber to the gas in the cylinder during the intake stroke, and an increased amount of heating of the gas in the cylinder. Thus, in a spark ignition engine, if the thickness of the thermal insulation layer is thickened too much, the amount of heating of the gas in the cylinder during the intake stroke is increased, the gas temperature at a compression end is increased, and knocking tends to occur more frequently.
JP 1-142246 A only discloses a range of the thickness of the thermal insulation layer (50 μm-220 μm), and does not consider is the change in the amount of heat transferred from the combustion chamber wall to the gas in the cylinder during the intake stroke depending on thermal properties of the thermal insulation layer, such as the thermal diffusivity. Thus, depending on the material of the thermal insulation layer, the amount of heating of the gas in the cylinder during the intake stroke may increase, and knocking may occur more frequently.
An advantage of the present invention is provision of a spark ignition engine in which the thermal efficiency is improved without worsening the knocking.